A direct modulation technique for driving a laser diode using an electric signal to generate an optical signal is available as an optical signal transmission technique. Direct modulation using laser diodes can generate optical signals with a simple configuration and is thus advantageous in reducing the cost of an optical communication system. Thus, direct modulation using laser diodes is used for short-range optical transmission systems. However, in the case of direct modulation using laser diodes, it is difficult to extend the transmission range. In long-range optical transmission systems, with the use of an external optical modulator, optical signals with a higher quality are generated. However, since such a configuration includes complicated and expensive components, the cost is not reduced.
A carrier signal from an oscillator is modulated using a data signal B by a multiplier, and an optical modulator outputs control light ECt, which is obtained by being modulated using the carrier signal modulated using the data signal B. A multiplexer multiplexes carrier light ES, which is continuous wave (CW) light, with the control light ECt from the optical modulator, and a nonlinear optical medium performs cross-phase modulation for the carrier light ES on the basis of the control light ECt. This technology is disclosed, for example, in Japanese Laid-open Patent Publication No. 2001-215603.
In direct modulation using laser diodes, a frequency chirp, which is a change in the emission light frequency based on the intensity of a driven electric signal, occurs. Thus, the waveform of a direct-modulated optical signal is degraded in a transmission path, and the transmission range is restricted.
In order to suppress the degradation in the waveform, trials have been conducted to extend the transmission range by applying pre-emphasis on driven electric signals and performing signal processing for received electric signals. However, there is a problem in that power consumption increases.
Furthermore, although optical signals generated using an external modulator can be transmitted over a long range, the driving voltage amplitude is higher than that for optical signals generated by direct modulation. In addition, since the insertion loss of the external modulator is large, electrical and optical amplifiers are provided, and this configuration increases the cost.